This article was written years before the movie and uses an OOP GW fortress.
http://larsen-family.us/~1066/helmsdeep.html I favor this product over many more realistic walls because the wall was designed with/for Old siege book and rules all years out of print. You can still find them on ebay from time to time.
I built my walls to fit with the scenario books.
I used the octagon tower mold and the wall builder#200 from Hirst Arts to match the Helms Deep GW model.
I Found a better design later.
Here is the early wall that Matches the height of the GW model.
You can match the stone pattern by engraving foam with a pen if you prefer.
In this photo you can see that the Deeping wall is made from several sections of the Original Mighty Fortress and a Wooden Tower also long OOP.
Note the other tower described in the book but not in the movie. The OOP Wooden tower represents the removable if destroyed section.
This drawing referenced in the above article was drawn on an envelope by JRR shows the Deeping wall as a wooden placade.